Many of the processes used for the treatment of hydrocarbons result in the formation of coke deposits on surfaces of the treatment vessel or reactor. Such processes include petroleum cracking such as thermal cracking or hydrocracking, visbreaking, refining, and upgrading. As described in commonly assigned pending U.S. patent application Ser. No. 771,205, filed Aug. 30, 1985, and in commonly assigned U.S. Pat. No. 4,648,964 of Leto et al. (1987), one type of apparatus which can be used in one or more of the above processes is a vertical tube reactor, i.e. a reactor which uses pressure obtained from the hydrostatic head of a column of fluid above the reaction zone. When high pressures are desired for the reaction zone, a large vertical extent is needed to develop the necessary hydrostatic pressure. In these cases, the vertical tube reactor can be disposed underground, such as in a well bore.
Deposition of coke in such a reactor is particularly troublesome because the difficulty of access renders conventional processes for removal of coke deposits especially burdensome. Removal of coke deposits is desirable because the coke inhibits heat transfer across the walls of the reactor vessel thus making heat exchange methods inefficient, decreases reactor volume, and can build up to such a degree that fluid flow through the reactor is inhibited or blocked.
A mechanical apparatus to physically dislodge or scrub coke particles has been employed in some reactors. U.S. Pat. No. 4,196,050 (1980) of Takahashi et al. describes a rotatable injection pipe for introduction of a scrubbing liquid with means for reciprocating motion. Such insertable devices are of limited value when the reactor is relatively inaccessible, such as in the case of a subterranean vertical tube reactor.
Other methods have relied on oxidation of the coke to remove coke deposits. U.S. Pat. No. 3,365,387 (1968) of Cahn et al. discloses decoking a thermal cracker by passing a mixture of steam and water through the reactor tubes at essentially the same temperature level as used for the thermal cracking. If water is used, it must be vaporized and super heated to about 700.degree. F. (371.degree. C.) prior to entering the section to be decoked. U.S. Pat. No. 3,054,700 (1962) of Martin discloses removing material from a shell and tube heat exchanger by introducing an oxygen-containing gas, possibly mixed with steam. U.S. Pat. No. 4,420,343 (1983) of Sliwka discloses thermal decoking of cracked gas coolers by introduction of a steam/air mixture. U.S. Pat. No. 4,376,694 (1983) of Lohr et al. discloses decoking surfaces of a cracking plant by admitting a steam and air gas mixture. U.S. Pat. No. 4,454,022 (1984) of Shoji et al. discloses removal of coke deposits in the gas passages of a thermal cracking apparatus by contact with a stream of oxygen-containing combustion gas.
All of these methods are impractical for use in decoking a reactor which is physically relatively inaccessible, such as a subterranean vertical tube reactor. The above methods all use a material which is in a gaseous state such as air, oxygen, or steam. In general, control of heat in the oxidation reaction has not been a major problem in above ground or otherwise physically accessible apparatus, even though such gaseous materials have a lower cooling capacity (i.e. thermal conductivity and heat capacity) than liquid phase materials. In such accessible apparatus, damage to the apparatus from overheating is avoided by external monitoring such as visually tracking the movement of the "hot spot" (the portion of the apparatus heated to relatively high, often irridescent temperatures). Visual monitoring is impractical in a subterranean or inaccessible apparatus. Use of temperature probes in a number sufficient for proper monitoring of wall temperature for a "hot spot" would be prohibitively expensive in a subterranean apparatus. Although such overheating can be controlled by reducing the rate of flow of the gaseous material, the consequent reduction in reaction rate can interfere with maintaining the minimum temperature necessary for establishing the desired reaction. Furthermore, reduction of the reaction rate increases the period during which the apparatus is off-line.
Gas phase decoking is also impractical in an inaccessible apparatus because gaseous material is inefficient for removing non-gaseous by-products and spalled coke from the reactor. If the gaseous material is used to blow out suspended by-product or spalled particulates, a relatively high flow rate is required to maintain the particles in suspension. When particle suspension becomes the determining factor with respect to gas flow rate, there is either inefficient utilization of the oxidizing reagent or an excessively long off-line period. The problem is particularly troublesome in a vertical reactor in which particulates must be not only suspended but lifted out of the reactor by the fluid flow. This difficulty is overcome in accessible reactors because any material which settles in the reactor can be manually removed, for example, by providing traps or drains. Such methods are impractical or expensive for removing material which has settled to the bottom of a subterranean or inaccessible reactor.
Some products of an oxidative decoking reaction can be corrosive depending upon the reactor materials of construction. When the decoking is accomplished using a gaseous material, the corrosive products can remain relatively concentrated or localized so that apparatus corrosion can be a significant concern. The occurrence of such corrosion is of particular concern when the reactor cannot be readily accessed for maintenance or replacement of parts.
In applications where the reactor is disposed far underground in order to achieve substantially elevated pressure, it is economically infeasible to supply steam to the reactor at the required pressure and temperature. The heat loss experienced by a long steam line would require extensive thermal insulation and/or auxiliary heating devices, either of which involves considerable expense in materials and design. In addition to heat loss experienced by an extended steam line, the high pressure developed at substantial distances below ground contributes to condensation of the steam.
U.S. Pat. No. 2,882,237 (1959) discloses that an aqueous solution of hydrogen peroxide can be used to remove carbonaceous resinous or gummy deposits, but only when the hydrogen peroxide is "activated" with ammonia. There is no suggestion in this patent of treatment at elevated pressure or the temperatures contemplated in the instant process.
Several references disclose use of an underground vertical reactor for various processes. U.S. Pat. No. 3,449,247 (1969) of Bauer discloses flowing refuse and fluid sewage in a subterranean vertical shaft to obtain the desired pressure for wet oxidation of the combustible waste materials. U.S. Pat. No. 3,464,885 (1969) of Land et al. discloses a subterranean reaction, particularly for digestion of wood chips. U.S. Pat. No. 4,272,383 (1981) of McGrew discloses a subterranean vertical reactor for accelerating chemical reactions including wet oxidation. It discloses the formation and use of Taylor Bubbles in which there is essentially plug flow of vapor phase "bubbles" in a liquid phase. It is particularly directed to the oxidation of sewage sludge.
U.S. Pat. No. 3,853,759 (1974) of Titmas discloses that adherence of materials to the apparatus walls of a subterranean vertical reactor can be deterred by providing for rotation of the tube or "liner". This process limits the oxidation reaction by restricting the process to the oxygen present in the material introduced, i.e. no additional oxygen is added. This reference does not disclose oxidation of adhered materials and thus does not recognize the problems of temperature control or precipitation of by-products discussed above.
U.S. Pat. No. 3,606,999 (1971) of Lawless discloses a vertical reactor for contacting solids, liquids and gases useful for utilizing physical, chemical, or thermal treatment under elevated pressure of continuously flowing streams which may contain suspended solids. This reference further discloses that any accumulation of sludge or fuel ash, or other insoluble materials below the reaction zone, can be removed continuously or intermittently if desired by a pump or siphon. This reference is not concerned with removing material which adheres to the reactor vessel.
These references are primarily concerned with oxidizing materials which are introduced into the reactor in an aqueous stream and the processes of these references are thus amenable to ready mixing of the oxidant and the material to be oxidized. When the oxidizable material is sewage sludge and waste streams as disclosed by McGrew, the purpose is to substantially oxidize all of the oxidizable materials. Therefore, oxygen is normally maintained in excess and the amount of sludge in the reactor is controlled. Additionally, excess heat is removed by a cooling system to increase throughput for a given reactor volume. These references provide no method for treating or reacting a material which is already in the reactor such as removing coke deposits from the walls of a reactor. These references thus do not recognize the attendant problems of thermal or corrosion damage to the reactor or accumulation of by-products. These problems are especially troublesome when the oxidant is not mixed with the material to be oxidized throughout the volume of the reaction zone, but rather the oxidation takes place substantially along the surface of a sheet of the material to be oxidized, e.g. a sheet of deposited coke. Further, the apparatus of each of these patents is intended to maintain flow of a fluid containing suspended particles as its normal mode of operation. Such apparatus is not necessarily operable in connection with a process which must maintain, under normal conditions, a flow of liquid with only minor amounts of suspended particles.
None of the known references discloses or suggests processes useful for removing deposits such as coke deposits which adhere to the surfaces of a relatively inaccessible reactor such as a vertical tube reactor disposed underground. None of the references recognizes or proposes solutions to the problems of heat damage from the exothermic oxidation of coke deposits, corrosive or acidic by-products resulting from coke oxidation, or retaining in suspension substantially insoluble coke oxidation by-products, particularly when the production mode involves treating a feed without substantial concentration of solid by-products. Therefore, there is a need to provide a method for removing coke from a hydrocarbon reactor which minimizes or eliminates the accumulation of by-products such as ash, particulates, or granules, and which minimizes or avoids thermal or corrosion damage to the reactor vessel.